In the manufacture of polymeric foams, such as, for example, polyurethanes, polyureas, phenol-formaldehydes and the like, a heat activated blowing agent is employed to provide the desired cell structure.
The term liquid material is understood to include any liquid material that can be converted into a polymer by a polymerization reaction. Of particular interest are polyurethane, polyurea and isocyanate polymers which are produced by contacting under reactive conditions suitable amounts of liquid material comprising a polyahl and an isocyanate.
The term polyahl is understood to include any compound containing active hydrogens in the sense of the Zerewitinoff test, see Kohler, Journal of the American Chemical Society, page 381, Volume 49 (1927). Representative active-hydrogen groups include --OH, --COOH, --SH and --NHR where R is H, alkyl, aryl and the like.
The term isocyanate is understood to include organic isocyanates and polymeric derivatives thereof useful in making polyurethanes, polyureas and polyisocyanurates, such as, aromatic, aliphatic and cycloaliphatic polyisocyanates. Exemplary compounds include toluene diisocyanate, diphenylmethane diisocyanate, polymeric diphenylmethane diisocyanate and mixtures thereof.
A crude polyisocyanate may also be used in the practice of this invention, such as, the crude toluene diisocyanate obtained by the phosgenation of a mixture of toluene diamines or the crude diphenylmethane diisocyante obtained by the phosgenation of crude methylene diphenylamine. The preferred undistilled or crude polyisocyanates are disclosed in U.S. Pat. No. 3,215,652, incorporated herein by reference. Derivatives of the above identified isocyanates, such as, prepolymers, are equally suitable for use in the present invention.
This disclosure relates to the manufacture of flexible and rigid foams as well as to systems employed in the manufacturing of reaction injection molded (RIM) and reinforced reaction injection molded (RRIM) products.
Flexible foam processing systems generally utilize polyester or polyether polyahls and toluene diisocyanate (TDI) or diphenylmethane diisocyanate (MDI) and small amounts of catalysts, surfactants and amines. Additionally, various blowing agents are utilized which generally consist of some of the following of methylene chloride and small amounts of R-12.RTM., for so-called mechanically blown foams, and water and small amounts of carbon dioxide for so-called chemically blown foam in which water is the primary blowing agent.
Rigid foam processing systems generally utilize polyether polyahls and MDI and small amounts of catalysts, surfactants and amines. Additionally, various blowing agents are utilized which typically consists of a combination of one to two percent of water and approximately 25 to 35 weight percent of chlorofluorocarbon (CFC). The CFC is normally R-11A.RTM..
RIM and RRIM foam processing systems generally utilize an amine terminated polyol, polyether or polyester polyahls and MDI and TDI and small amounts of catalysts and surfactants. Gaseous nitrogen blowing agent is normally utilized in the form of suspended bubbles or droplets in about a fifty percent by volume concentration at atmospheric pressure conditions. The compressed gas bubbles or droplets present during mold filling aid in complete filling of the mold and enhancement of surface characteristics of the molded product. RRIM differs from RIM, in that reinforcing fillers are added to the above described polyol or polyether polyester polyahls.
In the manufacture of foam products for use as insulation, chlorinated fluorocarbons (CFC's) specifically FREON.RTM. have been employed as the blowing agent because a very small and uniform cell structure results in the product and, in turn, provides an improved K factor. In order to eliminate the use of such CFC's, other blowing agents have been considered. While it has been customary to employ a chlorinated fluorocarbon (CFC) for this purpose, the cumulative effect on the ozone layer of the atmosphere has made it desirable to utilize environmentally friendly blowing agents. Gases including carbon dioxide, nitrogen, helium, ammonia, pentane, acetylene, the inert gases, air and mixtures thereof have been investigated. Unfortunately, the mere addition of some alternative blowing agents is difficult and heretofore has often not resulted in the manufacture of quality foam products.
Introduction of the blowing agent generally gives rise to a number of different types of phase behavior which depend on the miscibility characteristics of the blowing agent/liquid material combination at the given temperature and pressure. Essentially, insoluble blowing agents under high pressure, may give rise to the presence of both droplets of liquid blowing agent and bubbles and gaseous blowing agent in the liquid material, provided the temperature of the system remains below the critical temperature of the blowing agent and the pressure is high enough to cause liquefaction. Under similar conditions, a mixture will contain single, dissolved blowing agent molecules in addition to distinct droplets and bubbles of liquified and gaseous blowing agent, respectively.
In the production of polyurethanes, the blowing agent is mixed with a liquid material or reaction component of a two part system, e.g., polyahl or isocyanate, prior to polymerization. One of the problems associated with the use of non-CFC blowing agents has been their incorporation into the liquid material. For a given cell structure, it is known that a specific quantity of blowing agent must be present, however, the solubility and miscibility of the agent is an important factor with which the manufacturer must reckon.
To date, the patent art provides numerous examples of apparatus and methods for using various non-CFC blowing agents in a variety of liquid material components. For example, the introduction of an inert gas, such as, nitrogen, into a liquid reaction component of a reaction injection molding (RIM) system is taught by U.S. Pat. No. 4,157,427. In general, the gas is added to one of the precursors of a polyurethane by use of a sparger through which the gas is forced, under pressure. The sparger is described as a suitably sized and shaped porous rigid structure, to produce minute bubbles for better mixing, that is placed in a pipe through which the reactive component is circulated from the supply tank and then sent either to a mixing head or back to the supply tank.
U.S. Pat. No. 4,376,172 is directed toward a closed loop apparatus for controlling the addition of a gas to a liquid, such as, polyurethane precursor, in a RIM process. Additionally, means are provided for accurately measuring the amount of the gas that is added. The blowing agent or gas is added by means of a sparger which is in a stream of the reactant being recirculated from the supply tank and back to the supply tank.
Measurement of the amount of gas added to the polyurethane reactant is performed by trapping a volume of the gas-reactant mixture and holding it in a cylinder. A piston is then driven into the cylinder to check, by means of compressibility, the amount of gas which has been added.
U.S. Pat. No. 4,526,907 is directed toward a process and device for charging gas into at least one of the components combined to produce plastic foams. The reactant from one supply tank is piped through a circulation line which has a zone of compression that is higher in pressure than that in the supply tank. In this compression zone the foaming gas is added, and the mixture is subsequently forced through a throttle element to reduce the pressure before return to the supply tank. The patent also teaches that several different methods can be employed to determine the amount of gas in the gas-reactant mixture including density, partial pressure, the absorption of a beam of light, compressibility and solubility, but does not necessarily discuss means for doing so.
U.S. Pat. No. 4,906,672 is directed toward a method for the continuous manufacture of polyurethane foam. More particularly, it deals with the additions of small amounts of carbon dioxide to polyurethane-forming reactants which contain water as the primary blowing agent and teaches that the carbon dioxide is to be dissolved into one of the reactants well before being sent to the mixing head.
Introduction is performed under high pressure, preferably 75 to 900 psig (0.62 to 6.3 MPa), in a pipe, a sufficient distance from the mixing head that uniform entrainment is achieved upon traveling from the sight of impingement to the mixing head. Once the mixture reaches the mixing head, a nozzle or series of nozzles are employed to expand the carbon dioxide-reactant mixture; however, the patent teaches that the entrainment of bubbles is to be avoided. The patent does not contemplate the use of alternative blowing agents or mixtures thereof nor does it contemplate the addition of high concentrations of carbon dioxide as a major blowing agent component.
Finally, European Pat. No. 125,541-B discloses a devices for measuring the gas charging of a liquid component used for producing synthetic plastic foam, such as, a polyurethanes. It employs a measuring vessel, for receipt of a liquid sample periodically, and which communicates with an overflow vessel. By allowing the pressure in the measuring vessel to decrease to atmospheric, the gas laden component expands and overflows to the overflow vessel which allows density to be determined.
The prior art teaches determination of gas loading using measured density of the blowing agent/liquid material mixture at actual operating pressure (U.S. Pat. No. 4,157,427) or at ambient pressure. To this purpose, mixtures of polyahls and blowing agents are expanded either from a preset operating pressure to a second, lower set pressure (U.S. Pat. No. 4,376,172) or from a given preset operating pressure to atmospheric pressure (European Pat. No. 125,541-B). The latter invention utilizes equipment that is large, cumbersome and expensive. Moreover, at least part of the gas in the mixture will be lost from the system during expansion.
Thus, it should be apparent that although others have employed low boiling compounds, as blowing agents for polyurethane foam, apparatus and method have not been taught for the incorporation of a blowing agent into a liquid material, in precise amounts and bubble and droplet sizes so as to control the cell structure of the resulting foam, or for the precise measurement of the volumetric expansion potential of a mixture of blowing agent and liquid material, the liquefaction and solubility of the blowing agent in the mixture, or the determination of it's volumetric expansion potential therefrom. Moreover, previous apparatus and methods have not been successful in providing uniform incorporation of the blowing agent in the liquid material component, which has grossly affected the quality of the resulting foam product.
Another problem has been the accurate determination of the quantity of blowing agent actually incorporated into the liquid material component prior to reaction because escape of the agent during measurement leads to erroneous determinations and, in turn, the use of incorrect quantities of the blowing agent as corrections are made or not made.
Additionally, escape of blowing agents has precluded or made difficult or hazardous the use of flammable blowing agents.
It should also be clear from the above discussion that these methods will provide erroneous results where liquid blowing agent droplets and/or dissolved single blowing agent molecules remain in the liquid material after expansion. In such instances, relatively correct gas loading may be obtained but, the relationship between gas loading and density of the expanded product will be in error to the extent that expansion potential is not accounted for by the density measurement.